In a manufacturing process of various semiconductor devices, a silicon nitride film is formed as a gate insulating film of, e.g., a transistor. As a method for forming the silicon nitride film, there is proposed, e.g., a method for forming a silicon oxynitride film by introducing nitrogen into a silicon oxide film by a plasma process, in addition to a method for depositing a silicon nitride film by CVD (Chemical Vapor Deposition) (see, for example, Reference 1).
Meanwhile, with the recent trend of miniaturization of semiconductor devices, a gate insulating film is getting thinner, and it is required to form a thin gate insulating film having a film thickness of several nanometers. For the purpose, a research has been conducted to develop a method for forming a silicon nitride film by nitriding silicon directly.
As such a method for forming a silicon nitride film on a silicon substrate by introducing nitrogen into the silicon substrate directly, there has been proposed a method of heating the silicon substrate in a processing chamber filled with an ammonia gas and irradiating ultraviolet rays (see, for example, Reference 2). Further, though a method of directly nitriding the silicon substrate by forming a plasma of the ammonia gas with a parallel plate type plasma processing apparatus is also disclosed in Reference 2, there occurs a problem in such case that a damage of the silicon substrate is caused due to a very high energy of the plasma or a quality of the silicon nitride film deteriorates due to an unexpected reaction. Thus, Reference 2 proposes the nitirding method using the ultraviolet rays without using the plasma as a way to avoid the aforementioned problems that are caused when the silicon substrate is nitrided by the plasma.
Reference 1: Japanese Patent Laid-open Application No. 2001-274148 (Claims, etc.)
Reference 2: Japanese Patent Laid-open Application No. 2003-243387 (Claims, Paragraphs [0018]˜[0015], and FIG. 8)
As pointed out in Reference 2, when a nitride film is formed by directly nitriding silicon by the plasma, there may be incurred a so-called plasma damage that the quality of the nitride film deteriorates due to an implantation of high-energy ions into the film. This plasma damage would have an adverse influence upon characteristics of a device, e.g., a transistor, resulting in deterioration of device performance.
Further, when the silicon is plasma-nitrided directly, there is a problem that an N concentration decreases (N is lost from the film) or oxidation readily occurs with a lapse of time. In particular, as the thickness of the nitride film decreases, the possibility of the film quality deterioration due to the N loss or the oxidation increases. Thus, a formation of a stable nitride film becomes difficult.